This invention pertains to the aviation art and, more particularly, to a means for determining the position of an aircraft with respect to a runway.
The safe and efficient operation of commercial aircraft requires that the flight crew know the precise location of the aircraft with respect to the runway and that the aircraft is decelerating at a rate such that there is sufficient runway remaining for a controlled stop. Excessive decelerating forces on landing results in unnecessary wear to the aircraft's braking system as well as needless discomfort to the passengers. Should the decelerating forces be to low, however, there is the obvious risk of running off of the runway as well as the practical problem of the aircraft spending too much time on the runway thus delaying runway use by other aircraft.
Adverse weather conditions, such as fog, causing low visibility, and water or ice on the runway, conducive to skidding, can result in particularly hazardous conditions. In response to this, various approaches have been suggested for indicating to the flight crew the position of the aircraft with respect to the runway and the calculated distance to stop under present decelerating forces.
One such system uses wheel speed as a measure of airplane speed. This system suffers the obvious inadequacy that skidding occurs, particularly with slippery runways, thus resulting in significant errors.
A further system requires the placing of a special transmitter at the end of the runway. A receiver on board the aircraft processes signals from the transmitter site and provides an indication of the relative position of the aircraft with respect to the transmitter and, hence, the end of the runway. Further, the decelerating forces of the aircraft are monitored and a calculation is made to determine whether or not, under the present deceleration rate, the aircraft has adequate stopping distance. An obvious problem with this system is that it requires a special transmitter site to be located at the end of each runway.